Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment in order to remove toxins and excess fluids from their blood. To do this, blood is taken from a patient through an intake needle or catheter that draws blood from a blood vessel located in a specifically accepted access location (for example, a shunt surgically placed in an arm, thigh, subclavian, etc.). The needle or catheter is connected to extracorporeal tubing that is fed to a peristaltic pump and then to a dialyzer which cleans the blood and removes excess water. The cleaned blood is then returned to the patient through additional extracorporeal tubing and another needle or catheter. Sometimes, a heparin drip is located in the hemodialysis loop to prevent the blood from coagulating particularly in the dialysis filter. By way of background, as the drawn blood passes through the dialyzer, it travels in straw-like tubes within the dialyzer that serve as semi-permeable passageways for the unclean blood. Fresh dialysate solution enters the dialyzer at its downstream end. The dialysate surrounds the straw-like tubes and flows through the dialyzer in the opposite direction of the blood flowing through the tubes. Fresh dialysate collects toxins passing through the straw-like tubes by diffusion and excess fluids in the blood by ultra filtration.
It is known in the art to use an optical blood monitoring system during hemodialysis, such as the CRIT-LINE® monitoring system which is sold by the assignee of this application. The CRIT-LINE® blood monitoring system uses optical techniques to non-invasively measure in real-time the hematocrit and oxygen saturation levels of blood flowing through a hemodialysis system. In the CRIT-LINE® system, a sterile, single-use blood chamber is attached in-line to the extracorporeal tubing normally on the arterial side of the dialyzer. The blood chamber provides a viewing point for optical sensors during the hemodialysis procedure. As described in more detail below under the heading Detailed Description of the Drawings, the blood chamber used in the current system comprises a molded body made of clear medical grade polycarbonate. The chamber body along with the tube set and dialyzer filter are replaced for each respective patient. The blood lines and blood chamber are replaced for each treatment though the personal filter for each patient is often cleaned and reused over several treatments. The blood chamber provides an internal blood flow cavity, a flat viewing region and two viewing lenses: one being integrally molded with the body of the polycarbonate blood chamber and the other being welded into place. The LED photo emitters and the photodetectors for the optical blood monitor are contained on a sensor clip assembly that is clipped into place on the blood chamber over the lenses. Multiple wavelengths of visible and infrared light are directed through the blood chamber and the patient's blood flowing therethrough, and the resulting intensity of each wavelength are detected. The preferred wavelengths to monitor hematocrit are: a) about 810 nm (e.g. 829 nm), which is substantially isobestic for red blood cells, and b) about 1300 nm, which is substantially isobestic for water. The preferred wavelengths to monitor oxygen saturation are: a) about 660 nm, and b) about 810 (e.g., 829 nm). The system includes a sensor clip assembly having an LED emitter for each wavelength (e.g. 660 nm, 810 nm, and 1300 nm) and also a silicon photodetector to detect the intensity of the 660 nm and 810 nm light and an indium gallium arsenide photodetector to detect the intensity of the 1300 nm light. Signals from the photodetectors representing the intensity of each wavelength (660 nm, 810 nm and 1300 nm) are transmitted from the sensor clip assembly to a stand-alone electronic controller.
A ratiometric technique implemented in the CRIT-LINE® controller, substantially as disclosed in U.S. Pat. No. 5,372,136 entitled “System and Method for Non-Invasive Hematocrit Monitoring”, issued on Dec. 13, 1999 and assigned to the assignee of the present application, uses the information transmitted from the sensor clip assembly to calculate the patient's hematocrit and oxygen saturation levels in real-time. The hematocrit value, as is widely known in the art, is the percentage determined by dividing the volume of the red blood cells in a given whole blood sample by the overall volume of the blood sample. In a clinical setting, the actual percentage change in blood volume occurring during hemodialysis can be determined, in real-time, from the change in the measured hematocrit. Thus, an optical blood monitor, such as the CRIT-LINE® monitor, is able to non-invasively monitor not only the patient's hematocrit level but also the change in the patient's blood volume in real-time during a hemodialysis treatment session. The ability to monitor real-time change in blood volume facilitates safe, effective hemodialysis and the ability to manage body fluid retention.
The mathematical ratiometric model for determining the hematocrit value can be represented by the following equation:
                    HCT        =                  f          ⁡                      [                                          ln                ⁡                                  (                                                            i                      810                                                              I                                              0                        -                        810                                                                              )                                                            ln                ⁡                                  (                                                            i                      1300                                                              I                                              0                        -                        1300                                                                              )                                                      ]                                              Eq        .                                  ⁢                  (          1          )                    
where i810 is the infrared light intensity detected by a photodetector at 810 nm, i1300 is the infrared light intensity detected at 1300 nm and I0-810 and I0-1300 are constants representing the intensity incident on the blood accounting for losses through the blood chamber. The function ƒ[ ] is a mathematical function which has been determined based on experimental data to yield the hematocrit value. Preferably, the function ƒ[ ] in the above Eq. (1) is a relatively simply polynomial, e.g. a second polynomial. However, under some conditions, more complex fits such as a spline fit must be used.
The oxygen saturation level, or the oxygenated hemoglobin level, is determined using a ratiometric equation for red visible light at 660 nm and infrared light at 810 nm. The form of the preferred ratiometric model for determining oxygen saturation level is as follows:
                    SAT        =                  g          ⁡                      [                                          ln                ⁡                                  (                                                            i                      660                                                              I                                              0                        -                        660                                                                              )                                                            ln                ⁡                                  (                                                            i                      810                                                              I                                              0                        -                        810                                                                              )                                                      ]                                              Eq        .                                  ⁢                  (          2          )                    
where i660 is the light intensity of a photo receiver at 660 nm, i810 is the infrared intensity detected at 810 nm and I0-660 and I0-810 are constants representing the intensity incident on the blood accounting for losses through the blood chamber. The function g[ ] is a mathematical function determined based on experimental data to yield the oxygen saturation level, again preferably a second order polynomial, although it may be useful to use a pair of second order polynomials or spline fitting techniques depending on the hematocrit value.
In the prior art CRIT-LINE® system, the stand alone controller includes a display that provides real-time blood monitoring data for the patient undergoing hemodialysis. The controller contains a microprocessor that calculates the displayed data calculated by the ratiometric models discussed above. The stand alone electronic controller also controls the operation of the respective LED emitters and the detectors in order to multiplex the independent wavelength measurements. The stand-alone controller also contains signal processing and noise reduction hardware and software, as well as calibration software. Preferably, calibration is accomplished in the field by placing the sensor clip assembly onto a verification filter (made of layered plastic having known optical qualities) that is mounted to either the sensor cable or the casing of the controller. Calibration software within the controller verifies the calibration of the unit, or allows the user to field calibrate the unit to bring it back to factory calibration settings. In some instances, it may be necessary to return the unit to the factory for calibration.
Normally, as mentioned, the controller is provided as a stand alone unit. In some applications, however, it is desired to integrate the capabilities of the CRIT-LINE® into OEM hemodialysis equipment or the like. In these applications, one or more custom designed circuit boards containing the ratiometric models, the sensor control software, the signal processing hardware and software and the calibration software are placed in the OEM equipment to receive the signals from the sensor clip assembly.